meteorites. Firstly, that notwithstanding the small mass of the planet a very powerful volcano would still be required; and secondly, that we are obliged to assume that for every meteorite which could ever fall on the earth at least 50,000 must have been ejected. It is thus plain that if the meteorites have really been driven from some planet of the solar system, large or small, the volcano must, from one cause or another, be a very powerful one. As we must have a very powerful volcano in any case, we are led to inquire which planet possesses on other grounds the greatest probability in its favour. We have already spoken of the volcanoes on the earth, and we admit of course that at the present time they are utterly devoid of the necessary energy; but were the terrestrial volcanoes always so feeble as they are at present? Grounds are not wanting for the belief that in the very early days of geological time the volcanic energy on the earth was much greater than at present. We admit fully the difficulties of the view that the meteorites have really come from the earth; but they must have some origin, and it is reasonable to indicate the source which seems to have most probability in its favour. Grant for a moment that in the primæval days of volcanic activity there were some mighty throes which hurled forth missiles with the adequate velocity: these missiles would ascend, they would pass from the gravitation of the earth, they would be seized by the gravitation of the sun, and they would be compelled to revolve around the sun for ever after. No doubt the resistance of the air would be a very great difficulty, but this resistance would be greatly lessened were the crater at a very high elevation above the sea level. Some of these objects might perhaps revolve in hyperbolic orbits, and retreat never to return, while others would be driven into elliptic paths. Round the sun these objects would revolve for ages, but at each revolution-and here is the important point-they would traverse the point from which they were originally launched. In other words, every object so projected from the earth, would at each revolution cross the track of the earth. We have in this fact an enormous probability in favour of the earth as contrasted with Ceres. Only one Ceres-ejected meteorite out of every 50,000 could possibly cross the earth's track, while every earth-projected meteorite would necessarily do so. If this view be true, then there must be countless hosts of meteorites traversing space in elliptic orbits around the sun. All these orbits have one feature in common: they all intersect the track of the earth. It will sometimes happen that the earth is found at this point at the moment the meteorite is crossing; when this is the case the long travels of the meteorite are at an end, and it tumbles back on the earth from which it parted so many ages ago. It is well to emphasise the contrast between the lunar theory of meteorites (which we think improbable), and the terrestrial theory (which appears to be probable). For the lunar theory it would, as we have seen, be necessary that some of the lunar volcanoes should be still active. In the terrestrial theory it is only necessary to suppose that the volcanoes on the earth once possessed sufficient explosive energy. No one supposes that the volcanoes at present on the earth eject now the fragments which are to form future meteorites; but it seems probable that the earth may be now slowly gathering back, in these quiet times, the fragments she ejected in an early stage of her history. Assuming, therefore, with Tschermak that the meteorites have had a volcanic origin on some considerable celestial body, we are led to agree with those who think that most probably that body is the earth. We cannot forbear to mention one or two circumstances which seem to corroborate the view that the meteorites are really of ancient terrestrial origin. The most characteristic constituent of meteorites is the alloy of iron and nickel, which is almost universally present. Sometimes, as in the Rowton siderite, the whole object consists of little else, sometimes this alloy is in grains distributed through the mass. When Nordenskjöld discovered in Greenland a mass of native iron containing nickel, this was at once regarded as a celestial visitor. It was called the Ovifak meteorite, and large pieces of the iron were conveyed to our museums. There is, for instance, in the British Museum Close exami a most interesting exhibit of the Ovifak substance. nation shows that this so-called meteorite lies in a bed of basalt which has been vomited from the interior of the earth. Those who believe in the meteoric origin of the Ovifak iron are constrained to admit that shortly after the eruption of the basalt, and while it was still soft, this stupendous iron meteorite of gigantic mass and bulk happened to fall into this particular soft bed. The view is, however, steadily gaining ground that this great iron mass was no celestial visitor at all, but that it simply came forth from the interior of the earth with the basalt itself. The beautiful specimens in the British Museum show how the iron graduates into the basalt in such a way as to make it highly probable that the source of the iron is really to be sought in the earth and not external thereto. Should further research establish this, as now seems probable, a most important step will have been taken in proving the terrestrial origin of meteorites. If the Ovifak iron be really associated with the basalt, we have a proof that the iron-nickel alloy is indeed a terrestrial substance, found deep in the interior of the earth, and associated with volcanic phenomena. This being so, it will be no longer difficult to account for the iron in undoubted meteorites. When the vast volcanoes were in activity they ejected masses of this iron-alloy, which, having circulated round the sun for ages, have at last come back again. As if to confirm this view, Professor Andrews discovered particles of native iron in the basalt of the Giant's Causeway, while the probability that large masses of iron are there associated with the basaltic formation was proved by the researches on magnetism of the late Provost Lloyd. Besides the more solid meteorites which seem to be terrestrial, there can be no doubt that the debris of the ordinary shooting stars must rain down upon the earth in gentle showers of celestial dust. The evidence on this point is overwhelming. The snow in the Arctic regions has often been found stained with traces of dust which contains particles of iron. Similar particles have been found in the towers of cathedrals and under many other circumstances. There can be hardly a doubt that some of the motes in the sun beam, and many of the particles which good housekeepers abhor as dust, are really of a cosmical origin. In the famous cruise of the Challenger the dredges brought up from the depths of the Atlantic no wedges of gold, great anchors, heaps of pearl," but among the mud which they raised are to be found numerous magnetic particles which there is every reason to believe fell from the sky, and thence subsided to the depths of the ocean. Sand from the deserts of Africa, when examined under the microscope, yields traces of minute iron particles which bear the marks of having experienced a high temperature. The earth draws in this cosmic dust continuously, but the earth now never parts with a particle of its mass. The consequence is inevitable; the mass of the earth must be growing, and though the change may be a small one, yet to those who have studied Darwin's treatise on "Earth-worms," or to those who are acquainted with the modern theory of evolution, it will be manifest that stupendous results can be achieved by slight causes which tend in one direction. It is quite probable that an appreciable part of the solid substance of the earth may thus have been derived from meteoric matter, which in perennial showers descends upon its surface. CHAPTER XVIII. THE STARRY HEAVENS. Whence the Importance of the Solar System?-Home-View in Space-Other Stellar Systems-The Sun a Star-Stars are Self-Luminous-We see the Points of Light, but nothing else-The Constellations-The Great Bear and the Pointers The Pole Star-Cassiopeia-Andromeda, Pegasus, and Perseus-The Pleiades Auriga, Capella, Aldebaran-Taurus, Orion, Sirius; Castor and Pollux-The Lion-Boötes, Corona, and Hercules-Virgo and Spica-Vega and Lyra-The Swan. : In the previous chapters of this work we have considered the sun and the system of planets and other bodies which revolve in obedience to his potent sway. We have found in the survey of this system much to impress us, and much which is calculated to awaken our conceptions to the stupendous scale on which the heavens are constructed. It is, however, desirable that we should not disguise from our thoughts the circumstances which lead us to attribute to the solar system a position of such importance in the scheme of the universe. It is the fact of our residence on a planet belonging to the solar system, which gives the solar system its great importance in our eyes. The solar system is our abode in the universe. The other planets, Mercury, Venus, Mars, Jupiter, and Saturn are our neighbours; the moon is our inseparable attendant; while the sun is the lamp which gives us light, and the fire which gives us warmth. It thus happens that in our eyes the solar system has the interest and the familiarity of home. We are surrounded by it; its welfare is our welfare. We live our lives without perhaps bestowing many thoughts as to whether there may not be other systems besides ours, or as to whether our solar system may not be far surpassed in size and in splendour by many, perhaps we might say by countless, other systems in the universe. Y |